The transition metal-mediated cycloaddition reactions of alkynes are of great current interest. See, for example, N. E. Schore, Chem. Rev., 88, 1081 (1988). While palladium is among the most widely studied metals for such processes, multiple alkyne insertions, or insertion and subsequent cyclization back on to a preexisting aromatic ring usually predominates. For example, see P. M. Maitlis, J. Organomet. Chem., 200, 1616 (1980). However, a number of groups have recently reported the successful synthesis of fused heterocyclic ring systems employing transition metal catalyses. For example, Larock et al., J. Amer. Chem. Soc., 113, 6689 (1991) have reported the synthesis of various indoles via the palladium-catalyzed heteroannulation of internal alkynes (eq 1): ##STR1## wherein R.sup.1 is H, CH.sub.3, Ac or Ts, and R.sup.2 and R.sup.3 are phenyl or alkyl, among others. Synthetic routes to 2-mono-substituted benzofurans via heteroannulation of terminal alkynes have been reported by Castro et al., J. Org. Chem., 31, 4071 (1966); Arcadi et al., Synthesis, 749 (1989); and Kundo et al., J. Chem. Soc., Chem. Commun., 41 (1992). Castro et al. did not utilize a palladium catalyst and required the cuprous form of the acetylide (eq. 2). Castro et al. and Kundo et al. also utilized palladium with a copper cocatalyst (eq. 3). ##STR2##
A single example of Pd-catalyzed carboannulation to yield a 1,1,2,3-tetrasubstituted indene has been reported by R. Larock in Pure & Appl. Chem., 62, 653 (1990).
Therefore, a continuing need exists for effective methods to prepare multi-ring aromatic and heteroaromatic ring systems from relatively simple precursors.